Liquid dielectric compositions are used in various types of electrical apparatus, such as capacitors, condensor bushings, transformers, cables and the like. In the construction of capacitors, such as power factor correction capacitors, capacitor packs are formed of alternate layers of metal foil and a dielectric material which is impregnated with the liquid dielectric. It is desirable that the liquid dielectric have high dielectric constant, maintain a low dissipation factor and be compatible with other materials in the capacitor structure. Furthermore, the liquid dielectric must be capable of withstanding elevated and fluctuating temperature, pressure and voltage stress conditions over the entire operational life of the capacitor.
In the past, various types of liquid dielectric have been used in capacitors such as polychlorinated diphenyl, castor oil, mineral oil, silicone oil, and the like. Of the commonly used liquid dielectrics, polychlorinated diphenyl, such as trichlorodiphenyl, has been the most widely used in capacitor applications.
While the polychlorinated diphenyls produced an effective electrical dielectric system for a capacitor, their usage has provided certain ecological problems in that the polychlorinated diphenyls are virtually non-biodegradable with the result that if leakage or rupture occurs in the capacitor casing, or if the capacitor is discarded as obsolete, the polychlorinated diphenyl will remain as a pollutant in the environment and will not degrade to any appreciable extent even over extended periods of time.
Because of the ecological problems there has been increased activity in an attempt to develop a replacement for the polychlorinated diphenyls as a capacitor impregnant. Any substitute impregnant should be biodegradable and have dielectric properties and performance characteristics comparable to those of the polychlorinated diphenyl.
While higher molecular weight polybutene, having a molecular weight of over 800 has had some limited use in the past as a liquid dielectric in electrical cables, polybutene has not been used in high stress electrical capacitors for use in power factor correction applications for several reasons. First, the relative dielectric constant of polybutene is about 2.10 to 2.25 compared to about 5.9 for trichlorodiphenyl. In a capacitor using all paper, or a combination of paper and polyolefin film, as the solid dielectric material, a substantial amount of capacitance is lost due to the lower relative dielectric constant of the polybutene.
Secondly, polybutene tends to generate gas under conditions of aging at temperatures above room temperature and the development of gas tends to shorten the life of the capacitor by giving rise to corona discharges. It is also believed that the polybutene depolymerizes under high electrical stress, which can give rise to corona discharges and early dielectric failure. However, higher molecular weight polybutene has found use in the past in the manufacture of cables, due to the fact that with cables it is desirable to have low capacitance, and the stresses on the system are considerably lower.